At present, in the most simple solution intended for the application of only one label, application units of this type comprise a rotary shaft on which one or more application devices are mounted.
The shaft rotates about a main axis of rotation which is parallel with the axis of rotation of the carrousel. In this way, each application device cyclically passes from a label pickup position to a position for application of the label on the container. It should be noticed that the label is fed to the unit with glue already put on it. Upstream of the roller there is a label transfer blade which, after having glue put on it, picks up the label thanks to the action of the glue and feeds the label with glue on it to the application device.
For this purpose, the application devices comprise at least one pad made of elastically deformable material (such as a sponge) which forms an applying surface on which the label is retained during the passage from the pickup position to the application position. Moreover, the applying surface is pressed against the container to guarantee correct application of the label.
The preferred field for application of this invention is the labelling of bottles. In many cases, many different labels must be applied on each bottle: a main label on the front, a rear label on the back, a stamp or a neck label at the base of the neck, a band also on the neck, as well as a seal at the cap or cork.
Obviously, the simplest solution for applying such labels has for many years been the use of a different application unit for each type of label.
However, even in the labelling machines sector, as in many others, the need to minimise dimensions and costs is felt.
Consequently, over the years several types of machines were developed in which each labelling unit could simultaneously apply two different labels. That was possible thanks to the fact that, in general, two different labels are applied in such a way that they are vertically aligned with each other (for example a main label and stamp/neck label on one side, and a rear label and seal on the other).
Consequently, when the bottle is orientated for example in such a way as to allow application of the main label, it is also in the best position for application of the upper neck label/stamp or the seal.
Therefore, over the years several machines have been developed in which each application unit was doubled, that is to say, comprised one or more lower application devices (intended for applying the main label or the rear label) and one or more upper application devices (respectively intended for applying the stamp/neck label and the seal).
The movement of such double units was always guaranteed by a single rotary shaft.
However, that proved to be a problem as regards synchronisation of the speeds between the application device and the containers.
As is known, to be able to apply labels in the best possible way, the applying surface and the surface of the container must be brought into contact (obviously with the label interposed between them) with a rolling movement of one on the other without slipping.
However, since the bottle neck has a diameter which is smaller than the bottle body, during bottle rotation (about the axis of rotation of the carrousel or about its own central axis by means of rotation of the supporting plate), the surface of the neck moves with a tangential speed which is less than that of the surface of the body.
Consequently, for ideal application of the labels, on one hand the upper application devices would need to have, at their applying surface which is pressed against the bottle, a tangential speed equal to that of the surface of the bottle neck, and on the other hand the lower application devices would need to have, at their applying surface which is pressed against the bottle, a tangential speed equal to that of the surface of the bottle body, and therefore a tangential speed greater than that of the upper application devices.
However, said requirement clashes with the need, dictated by dimensions, to move the lower and upper application devices using a single rotary shaft with a single rotational speed. In order to be able to correctly adhere to the bottle, the applying surface of the lower devices is positioned at a radius which is less than that of the upper devices. As a result, the applying surface of the lower application devices moves with a tangential speed which is less than that of the upper devices.
However, since the differences are not too noticeable, in the most common solutions attempts were made to solve that problem by adjusting the machine for an intermediate speed between those which are optimum respectively for labelling the body and the neck, basically accepting small slipping movements between the labels and the bottles.
Obviously, said solution is not suitable for high quality labelling operations.
In other solutions attempts were made to overcome the problem by modifying the structure of the upper application devices, making them free to oscillate relative to the shaft at the moment of contact against the bottle neck. In this way, when the applying surface makes contact with the bottle, its tangential speed adapts to that of the bottle. An elastic return then causes it to return to the normal position at the end of application.
However, even that solution is not without disadvantages, since due to the elastic movements of the various parts it cannot be used in equipment which has high productivity and therefore a high movement speed.
Examples of the latter type of machines as well as other types of equipment for the simultaneous application of two labels are described in U.S. Pat. No. 4,090,913, DE 2531737, U.S. Pat. No. 4,581,097, U.S. Pat. No. 2,723,042, GB 1080370, GB 952231, GB 814975, U.S. Pat. No. 4,118,269, U.S. Pat. No. 3,116,193, U.S. Pat. No. 2,631,748, DE 1022150, FR 2581363 and FR 2507567.